"Roll up your sleeves of seriousness. Do not give up one span of land that you have liberated," spokesman Abu Muhammad al-Adnani says. "March to Baghdad... We have a score to settle."

The nearly 17-minute recording was flagged and translated by the Washington-based SITE Intelligence Group, which monitors jihadist websites. It was not immediately possible for NBC News to independently verify its authenticity.

The rapid advance of the Sunni fighters - who seek to establish an Islamic emirate stretching across the Iraq-Syria border - has ignited fears of a sectarian bloodbath, prompting the U.S. and United Nations to express grave concerns and urge unity from Iraq's leadership.

Prime Minister Nouri al-Maliki - who has been fiercely criticized for failing to stem a rising tide of violence and for implementing what are considered to be broadly sectarian policies under his Shiite-led government - has requested a state of emergency for the county.

Lawmakers are expected to convene for an emergency session Thursday to consider that request, according to The Associated Press.

Officials told NBC News that the Iraqi government has asked the U.S. military to fly combat missions with manned aircraft and drones over military encampments – but so far the U.S. has not agreed.

"We’re not going to war in Iraq anytime soon," one official told NBC News.

The prime minister's leadership was mocked in the recording from ISIS, with Adnani slamming Maliki as a "liar" and nothing more than "an underwear salesman."

"What have you done to your people," Adnani says. "What do you know about policy, leadership, and military command?"

While the recording is not the first time ISIS leaders have urged a march on Baghdad - the group's leader reportedly issued the same decree in a January audio recording - the orders have taken on greater magnitude in light of the fresh offensive that brought down Mosul and Tikrit.

The White House has pledged to stand by Iraq's leaders, saying that the insurgents' actions in Mosul and its surrounding areas show "once again that these extremists seek nothing but death and destruction."

The United Nations Security Council has scheduled a meeting for later Thursday for a briefing from its special representative for Iraq.

Iraq's ambassador to France called on the U.N. Security Council to approve extra military aid for Baghdad, including air and drone support, when it meets, Reuters reported.

"We need equipment, extra aviation and drones," Fareed Yasseen said, when asked on France Inter radio what Iraq wanted from the Council.

U.N. Secretary General Ban Ki-moon has condemned the upsurge in violence and urged the international community to unite in showing solidarity with Iraq in confronting the “serious security challenge.”

“Terrorism must not be allowed to succeed in undoing the path towards democracy in Iraq as determined by the will of the Iraqi people,” he said in a statement late Wednesday.

Many in and outside of Iraq have expressed incredulity that Iraq’s security forces fled their posts in the face of the insurgents’ assault, littering the ground in Mosul with discarded uniforms and weapons as they abandoned their positions.

Analysts say the million-strong U.S.-trained Iraqi army greatly outnumbers the militants, but has failed to put up any serious resistance to the insurgents’ drive and mostly evaporated amid incoming attacks.

On Thursday, Iraqi Kurds seized control of the northern oil city of Kirkuk following clashes there that again forced a retreat by Baghdad’s security forces, according to Reuters.

"The whole of Kirkuk has fallen into the hands of peshmerga," Jabbar Yawar, a spokesmen for the fighters from Iraq’s autonomous Kurdish north told the news agency. "No Iraqi army remains in Kirkuk now."

Kurdistan has largely escaped the violence that has convulsed Iraq since the U.S. invasion. In recent months, Less than three years after the U.S. troop withdrawal, violence in Iraq has reached levels not seen since before the invasion.

Jim Miklaszewski of NBC News contributed to this report.

First published June 12 2014, 1:17 AM

Cassandra Vinograd

Cassandra Vinograd is a Senior Writer and News Editor. Before joining NBC News, she worked as a London-based correspondent for The Associated Press and specialized in politics, foreign affairs and defense.

Vinograd previously worked as an editor for The Wall Street Journal in Brussels and London.

She has reported extensively from Afghanistan and on West Africa and the Middle East.